Project Summary Regulatory T cells (Tregs) are a population of CD4+FoxP3+ T cells that are necessary for immune homeostasis and the prevention of spontaneous autoimmunity. The phosphtidylinositol-3 kinase (PI3K) pathway influences the growth, proliferation and survival of many cell types, and we have recently shown that control of PI3K activity by PTEN is critical for the suppressive function and lineage stability of Tregs. Mice that specifically lack PTEN in the Treg compartment (Pten-?Treg) develop an autoimmune lymphoproliferative disease as well as an expanded population of Tregs with unstable FoxP3 expression. Surprisingly, young unmanipulated Pten-?Treg mice also developed expanded populations of germinal center (GC) B cells, T follicular helper (Tfh) cells, and T follicular regulatory (Tfr) cells, accompanied by large amounts of class switched immunoglobulins and autoantibodies. This suggests that control of PI3K activity may be particularly important in the recently described subpopulation of T follicular regulatory (Tfr) cells, which regulate GC activity and antibody production. The aims in this proposal will focus on understanding the how the PI3K signaling pathway influences the maintenance and function of Tfr cells in vivo, potentially uncovering novel therapeutic targets that can be used to modulate control of antibody production. Pten-?Treg mice have an expanded population of Tfr cells yet develop significant humoral abnormalities, suggesting that PI3K activity alters Tfr cell maintenance and suppressive function in vivo. In Aim 1 of this proposal we will assess how PI3K activity in Tregs influences the differentiation and gene expression profiles of Tfr cells. In Aim 2 will focus on how PI3K activity in Tfr cells influences the regulation of GC activities in both unmanipulated and immunized mice. Knowledge of the regulatory mechanisms that control humoral immunity should translate into clinical approaches that will be important both in the context of disease treatment as well as prophylactic vaccination strategies. A better understanding of how Tfr cells function to regulate the GC, and how signaling pathways like PI3K can be utilized to control their function pharmacologically, may allow us to more efficiently treat and/or prevent antibody-mediated diseases as well as improve vaccination strategies.